Method for reinforcing a double-shell structure

ABSTRACT

The invention concerns a method for reinforcing a double-shell structure in which a cellular core ( 2 ) is arranged between a first shell ( 5 ) and a second shell ( 6 ). In the method according to the invention, at least one cavity is formed in the double-shell structure from an opening in the first shell into the cellular core ( 2 ) so that the cavity substantially extends through the cellular core ( 2 ) to the second shell. Fixative ( 8 ) is injected into the cavity. A stiff bar ( 7 ) is introduced into the cavity, whereupon excess fixative ( 8 ) is allowed to pass out of the cavity. Finally, the fixative left in the cavity is cured.

TECHNICAL AREA

The invention concerns a method for reinforcing a double-shell structurein which a cellular core is arranged between a first and a second shell.

STATE OF THE ART

A double-shell structure with composite or sheet metal shells oftencontains a spacer material of the cellular core type. For fabricationreasons, a cellular core is often made up of a plurality of units of thecellular core type, which are glued together to form the cellular core.The joints can become overloaded during fabrication or operational use,so that the units are at risk of separating from one another at one or aplurality of segments along the joints. This degrades the strength ofthe cellular core substantially.

U.S. Pat. No. 5,190,611 discloses a method for repair of compositestructures. The method is used when there is an undesired opening in apanel of composite material. Material is removed from the panel in theregion surrounding the opening and the prepared opening may extend allthe way through the panel. An insert is positioned in the preparedopening and bonded to the surface. This method is suitable for repair ofcomposite structures comprising layers of reinforcing fibers within aresin matrix. However, the method disclosed is not applicable forreinforcing or repairing so called double-shell structure and theparticular problems that arise from cracks between the units in thecellular core.

In order to reinforce or repair a cellular core inside an article havinga double-shell structure, holes must be made in one of the shells. Theholes made for the purpose of such repairs must subsequently berepaired. If the holes lead to a weakening of the shell, the shell mustbe reinforced so that it can bear its loads. Such repairs are usuallyextremely expensive, and it is also difficult to achieve sufficientstrength in both the cellular core and the shell in which the holes havebeen made.

No effective methods currently exist for repairing glue joints. Themethods that are used at present are based on cutting out a portion ofthe cellular core material at the glue joint, after which a repairsection is inserted and fixed by glue. Because it is difficult to hardenthe repaired area to the same strength as the undamaged cellular core,such repairs lead to structural degradation.

DESCRIPTION OF THE INVENTION

The object of the invention is to provide a method for reinforcing adouble-shell structure having an intermediate cellular core. Thecellular core may be a honeycomb structure, a foam structure or anyother type of cellular structure suitable as spacer material. Thereinforcement is achieved by means of a method in which a cavity isformed in the double-shell structure from an opening in the first shellinto the cellular core so that the cavity substantially extends throughthe cellular core to the second shell.

Fixative is injected into the cavity, after which a stiff bar whoseshape conforms to the cavity is introduced into same, whereupon theexcess fixative is allowed to pass out of the cavity and is removed. Thefixative left in the cavity hardens.

The invention pertains in particular to a method for reinforcing acellular core that comprises a first and a second unit of the cellularcore type arranged next to one another and joined by means of a gluejoint, whereupon the cavity can extend through the first unit and intothe second unit.

In another embodiment of the invention, the cavity is realized at anangle falling within the range of 30-60° relative to the horizontalplane. The cavity preferably consists of a longitudinally extended drillhole with a cylindrical cross-section.

According to yet another embodiment of the invention, said elementconsists of a bar with a non-cylindrical cross-section, whereupon atleast one passage for the fixative is realized after the bar isintroduced into the cavity.

The described method can be used to reinforce glue joints between afirst and a second unit in a cellular core. The method can also be usedto repair broken joints, e.g. in double-shell articles. The methodaccording to the invention is also suited for repairing locally damagedportions of a cellular core, or to reinforce/stiffen cores indouble-shell structures. The method is used for repairs both in theworkshop and in the field, i.e. proximate to the area in which thedamaged article is normally used.

One advantage that the repair method according to the invention has overthe repair methods currently in use is that the making of cavities inthe shells can be limited. The weakening of the shell is thus veryminor. This means that the repair of the cavity in the shell is simple,and that reinforcement may not need to be carried out, but rather itsuffices to seal the cavities to prevent moisture from penetrating intothe core. The method is also fast, in that repairs are accomplished viaa single hardening step. The method can also provide increasedstiffness/strength in comparison with the original, undamaged material.

BRIEF DESCRIPTION OF FIGURES

FIG. 1 shows a side view of a double-shell structure repaired using themethod according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a side view of a double-shell structure 1 repaired usingthe method according to the invention. The double-shell structureincludes a first shell 5 and a second shell 6 on each side of a cellularcore 2. The first and second shells 5, 6 are preferably compositematerial or sheet metal. The cellular core consists of any materialsuitable in a honeycomb structure, foam structure or other type ofcellular structure. The figure shows a bar 6 made of material suitablefor the purpose, such as carbon fiber, which has been fixed in a cavitythat extends through the cellular core 2 which, in the figure, comprisesa first unit of the cellular core type 3a and a second unit of thecellular core type 3 b. The bar has been fixed in place by means of afixative 8.

The opening to the cavity is realized in one of the surfaces 5, 6 of thedouble-shell structure. In the embodiment shown in the figure, thecavity is realized through a first shell surface 5 in a first unit ofthe cellular core type 3 a and continuing on into a second unit of thecellular core type 3 b; both the units 3 a, 3 b can be made of the samematerial or different materials, e.g. materials of different densities.The cavity extends through the glue joint 4. Because the cavity isrealized on the inside of the double-shell structure, the repair is notvisible on the outside of the double-shell structure. The cavity in theembodiment shown has not been realized in through-passing fashion. Oneadvantage of this embodiment is that repairs to the shell layer need becarried out on only one of the shell surfaces 5, 6. It is of course alsopossible to realize the cavity in through-passing fashion and, after thebar 7 has been fixed in place, to repair and seal both the first andsecond shell 5,6. The cavity can consist of a drill hole through thefirst shell 5 and the cellular core 2. Other types of mechanicalmachining to create the cavity are of course also possible, as are othermachining methods by means of which a cavity of the specified shape canbe formed. When drilling is used, a cavity with a cylindricalcross-section is obtained.

A bar 7 of, e.g. carbon fiber is given a longitudinal shape that canexceed the depth of the drill hole. The bar 7 can also be fabricated ata length matched to that of the drill hole, so that the bar 7 need notbe shortened after fixation. It is often advantageous to make the barsfrom the same material as the surrounding shell structures, althoughother material choices are of course also possible.

The bar 7 is preferably given a non-cylindrical cross-section, e.g. anangular cross-section with dimensions such that the bar can beintroduced into the cylindrical drill hole. Because the cross-section ofthe bar differs from that of the drill hole, four channel-likepenetrations will be present between the bar and the drill hole when thebar is introduced into the drill hole. Other cross-sections are ofcourse also possible in connection with the design of the bar. However,it is important that the cross-section be chosen so that a penetrationfrom the bottom of the drill hole to its opening is obtained even whenthe rod 7 is introduced into the drill hole.

Paste glue or other fixative 8 is injected into the cavity, filling itentirely. The bar is then pushed down into the cavity. The angularity ofthe bar enables excess glue to be forced up to the surface along thebar. The four channel-like penetrations are filled with glue, and theglue is disposed around the bar in this way. This is favorable from astrength standpoint. The excess glue is removed. The same effect can beachieved by realizing a through-passing channel in the bar.

When the glue 8 has hardened, the bar is cross-cut and ground flush withthe shell. The bar can also be prefabricated at a length that enablesthe entire bar to reside beneath the shell surface. The first shell 5 issubsequently sealed and optionally reinforced to the required level.

The method will now be described in connection with the repair of ahatch on an airplane. The hatch is examined radiographically, and thelengths and positions of any cracks are noted. Appropriate locations forthe cavities are marked out on a first surface. To avoid additionaldamage to the cellular core, a short grinding pin with a diamond surfaceis used to penetrate through the first shell. Cavities are then drilledthrough the core and the damaged porous material. A drilling jig issuitable for this purpose. The cavities are vacuumed clean, after whichthe surfaces around the cavities are protected with masking tape.

A carbon fiber bar with dimensions matching the cavity is prepared. Thebar can have an angular cross-section, so that one or a plurality ofchannels is formed between the bar and the inner surface of the cavitywhen the bar is introduced into the cavity. The bar is coated with HysolEA9394 or some other suitable fixative. The same fixative is injectedinto the cavity. It is important that fixative be applied all the waydown to the bottom of the cavity. Following completed injection of thefixative, the bar is introduced. The introduction of the bar must occurat different speeds, depending on the type of fixative chosen. WhenHysol EA9394 is used, it important that the bar be introducedimmediately after injection is completed. The injection of a bindingagent creates pressure in the cellular core, which tends to force theHysol back out of the cellular core. The carbon rod is insertedimmediately after injection is completed in order to prevent the Hysolfrom being forced back out.

After the bar is in place, the structure is allowed to harden at roomtemperature for a period of time that depends on the fixative chosen.The method can be repeated for a plurality of drill holes. Oncehardening has been completed, the masking tape can be removed and anyexcess fixative ground off. The first shell is sealed by coating it withsome type of composite plastic, which can harden under pressure at roomtemperature or some higher temperature. The surface is subsequentlysmoothed.

The method can be used to stiffen or reinforce a cellular core. Oneexample is the need for a new fastening point on an existing sandwichstructure. The new fastening point may require a stiffer or strongercore than was originally chosen. Using the proposed method, the core canbe reinforced beneath the fastening point with minimal intervention.

The method can also be used to reinforce existing joints in a cellularcore 2. The method can further be used to repair broken joints, e.g. inconnection with the fabrication of double-shell articles or the repairof locally damaged components in a cellular core.

List of Reference Designations

-   Double-shell structure 1-   Cellular core 2-   First unit of cellular core type 3 a-   Second unit of cellular core type 3 b-   Glue joint 4-   First shell 5-   Second shell 6-   Bar 7-   Fixative 8

1. A method for reinforcing a double-shell structure in which a cellularcore is arranged between a first shell and a second shell, methodcomprising the steps of: forming at least one cavity in the double-shellstructure from an opening in the first shell into the cellular core sothat the cavity substantially extends through the cellular core to thesecond shell; injecting a fixative in the cavity; introducing an elementinto the cavity, whereupon excess fixative is forced to pass out throughthe opening and be removed; and curing the fixative left in the cavity.2. A method according to claim 1, wherein the cellular core comprises afirst and a second unit of the cellular core type that are arranged nextto one another and joined by means of a glue joint, whereupon the cavityis arranged to extend through the first unit and into the second unit.3. A method according to claim 2, wherein the cavity is disposed at anangle falling within the range of 30-60° relative to the horizontalplane.
 4. A method according to claim 1, wherein said element is a barwith a non-cylindrical cross-section, whereby at least one passage forfixative is formed upon the introduction of the bar into the cavity. 5.A method according to claim 4, wherein the bar is relatively rigid.
 6. Amethod according to claim 5, wherein the relatively rigid bar is a rod.7. A product formed by the process described in claim 1.